1. Field of Industrial Uses
This invention relates to a method for driving a photoelectric device, especially a two-dimensional photoelectric device, and to a method for driving an image intensifier (II) using the photoelectric device.
The image intensifier intensifies the contract of an image of an extremely small quantity of light. This invention relates to a method for driving a proximity-type image intensifier comprising a two-dimensional photoelectric device comprising a photocathode in the side of the electron input electrode of a microchannel plate (MCP) for multiplying electrons, and a phosphor screen arranged in the side of the electron output electrode of the MCP.
2. Related Background Art
It has been conventionally proposed to give the proximity-type image intensifier the high-speed shuttering function (gating function). To this end, in the most common driving method a gate is closed by constantly applying a positive voltage between the photocathode and the input electrode of the MCP and, by applying a negative pulse voltage (acceleration voltage) at a required timing, the gate is opened during the time. In a one-step advanced driving method, for the purpose of varying the gate opening time (shutter opening time) negative voltages having a time lag with respect to each other are applied respectively to the photocathode and the input electrode of the MCP (Technical Report, The Institute of Television Technology, Vol. 11, No. 28, pp 31-36 (Nov. 1987)).
But the former driving method has problems that ringings and iris effect take place.
That is, it is difficult that the usual photocathode matches the driving impedance of an acceleration negative pulse voltage applied thereto. As a result, ringings take place in the negative pulse voltage. FIG. 1(S) shows the ringings of the negative pulse voltage. The gain between the photocathode and the input electrode of the MCP is substantially linear with respect to an applied voltage to the photocathode. As a result, the ringings of the negative pulse voltage directly affect the intensity of the output radiation from the phosphor screen. That is, as shown in the waveforms a, b of the output radiation intensity of FIG. 1(B), the gate is adversely opened by the ringings of the negative pulse voltage.
In addition, the usual photocathode has high surface resistance, and consequently, even when a voltage having a sharp rise, such as a negative pulse voltage, is applied, the voltage does not immediately arrive up to the center of the photocathode due to its RC time constant. As a result, the gate has "iris effect", which opens the gate from the outer circumference of the photocathode radially inward toward the center thereof. The iris effect restricts a substantial minimum gate opening time, specifically, at present, up to around 3 ns.
In the latter method, in which different negative voltages which are delayed in timing from each other are applied to the photocathode and the input electrode of the MCP, compared with the former method the ringing is less affective, but the iris effect still takes place.
To prevent the iris effect, a metal is vaporized on the photocathode to lower the surface resistance. But in this case, the transmittance of the photocathode is lowered, and a new problem of poor sensitivity occurs.
These problems exit also in the two-dimensional photoelectric device comprising a proximity-type image intensifier having the phosphor screen omitted.